1. Field of the Invention
The present invention relates to an engine mount for vibration-isolating support of a power unit on a body of an automobile, and more particularly to a fluid-filled engine mount of improved construction, that utilizes the flow action of non-compressible fluid sealed in its interior to produce effective vibration damping action against vibration of multiple, wide frequency ranges, such as engine shake and idling vibration.
2. Description of the Related Art
There are known in the art, as an engine mount for use in automotive vehicles, an engine mount of fluid-filled design having a first mount fixture and a second mount fixture for respective attachment to either the power unit or the vehicle body, a rubber, elastic body elastically connecting the fixtures, a pressure receiving chamber whose wall is partially formed by the rubber elastic body, and an equilibrium chamber whose wall is partially formed by a readily deformable flexible layer. A non-compressible fluid is sealed within the pressure receiving chamber and equilibrium chamber, and an orifice passage is provided for a communication between the two chambers.
Typically, an automotive engine mount is required to meet a variety of vibration to be damped whose frequencies differ depending on driving conditions. However, vibration-damping action based on flow action of fluid flowing through the orifice passage is limited to a relatively narrow frequency band to which the orifice passage has been pre-tuned.
The present assignee has been proposed in JP-A-8-270718, a pneumatically switchable type, fluid filled engine mount including: a first orifice passage tuned to the frequency of a first vibration to be damped; a second orifice passage tuned to the frequency of another vibration to be damped in a higher frequency band than the tuning frequency of the first orifice passage; a valve member for opening/closing the second orifice passage; and a pneumatic actuator that utilizes air pressure exerted from the outside to drive the valve member. In this engine mount, the valve member is driven so that the second orifice passage is placed in the closed state by means of atmospheric pressure exerted on the pneumatic actuator from the outside, or so that the second orifice passage is placed in the open state by means of negative pressure exerted from the outside, whereby the first orifice passage and second orifice passage are made to function selectively depending on the vibration to be damped, thereby producing the desired vibration damping action.
In recent years, an even higher level of vibration damping performance has come to be required, and in some instances the engine mount taught in JP-A-8-270718 is not sufficient to affords the required level of vibration damping performance. One required characteristic is damping ability against high frequency vibration such as running booming noise, which can be a problem during driving. Another required characteristic is damping ability against low frequency vibration such as engine shake, which can be a problem during driving. With regard to this latter low frequency vibration, damping ability against two types of vibration, namely low frequency; large amplitude vibration which is a problem when driving over speed bumps or the like, and low frequency, small amplitude vibration which is a problem during normal driving.
To coop with the first instance the required characteristic of damping ability against high frequency vibration, the present applicant has been proposed, as taught in JP-U-2-25749, to dispose a rigid movable plate in the partition wall that divides the pressure receiving chamber and the equilibrium chamber so that the plate is displaceable over a very small distance, whereby when high frequency vibration above the tuning frequency band of the first orifice passage and second orifice passage is input, pressure fluctuations in the pressure receiving chamber are absorbed by a very small level of displacement of the movable plate, creating lower dynamic spring.
However, when such a movable plate is employed, there is a risk that pressure fluctuations occurring in the pressure receiving chamber will be absorbed by displacement of the movable plate, even at times of input of small amplitude vibration in the low frequency range. This makes it difficult to ensure adequate fluid flow level through the first orifice passage tuned to the low frequency range, resulting in the difficulty in achieving sufficient attenuating action on low frequency, small amplitude vibration. Additionally, the rigid movable plate needs a gap on the outer peripheral side of the movable plate in order to permit slight displacement thereof, likely permitting a leak of fluid pressure from the pressure-receiving chamber to the equilibrium chamber through the gap. As a result, pressure fluctuations occurring in the pressure receiving chamber during input of low frequency, small amplitude vibration or medium frequency, medium amplitude vibration can leak, making it difficult to assure adequate fluid flow level through the first and second orifice passages, with the resultant problem of a decline in damping ability against the low amplitude component of engine shake and the medium amplitude component of idling vibration.
With the foregoing in view, the present application has also proposed, as taught in JP-A-9-310732, to dispose a movable film consisting of thin rubber film in place of the rigid movable plate, whereby liquid pressure absorbing action based on elastic deformation of the movable film provides lower dynamic damping of high frequency vibration above the tuning frequency range of the second orifice passage.
However, it was found that, just as with the movable plate mentioned earlier, when such a movable film was employed, there was a risk that elastic deformation of the movable film would absorb pressure fluctuations in the pressure receiving chamber, including low frequency, small amplitude vibration and medium frequency, medium amplitude vibration. This makes it difficult to provide sufficient damping action against engine shake and idling vibration.
To address this problem, the applicant has further proposed, as taught in JP-A-5-118375, to provide a working air chamber to the opposite side of the movable film from the pressure receiving chamber, whereby in association with negative pressure exerted on the working air chamber from the outside, the movable film is made to undergo constraining deformation, limiting the extent of elastic deformation thereof. Namely, by ensuring a sufficiently high level of fluid flow through the first orifice passage and second orifice passage during input of engine shake or idling vibration, by means of limiting the extent of elastic deformation of the movable film to suppress absorption of pressure fluctuations in the pressure receiving chamber, damping action of engine shake or idling vibration can be advantageously realized.
However, in the fluid-fluid engine mount disclosed in JP-A-5-118375, when limiting the extent of elastic deformation of the movable film, in consideration of phase difference, movable film free length, working air chamber size etc., negative pressure is exerted on the working air chamber, and the movable film undergoes a high level of undergo constraining deformation. This makes the mount control system and overall construction complicated. Thus, with the engine mount in question there are appreciable disadvantages in terms of production efficiency and production costs, another inherent problem is the difficult of installation in an automobile.
Further, the present applicant has made another proposal, while focusing on the fact that vibration which poses a problem in the high frequency range typically has small amplitude. Namely, as taught, for example, in JP-A-2000-310274 and JP-A-2001-200884, the present assignee has proposed to dispose a slightly displaceable removable plate extending at a generally right angle to the direction of opposition of the pressure receiving chamber and the equilibrium chamber with respect to a partition member, whereby pressure fluctuations produced in the pressure receiving chamber by input of vibration in the high-frequency band above the tuning frequency band of the first orifice passage and second orifice passage can be absorbed by minute displacement of the movable plate, producing a low dynamic spring rate.
Where this kind of movable plate was employed, however, there was a risk that pressure fluctuations produced in the pressure receiving chamber would be absorbed by displacement of the movable plate, even during input of vibration in the low- to medium-frequency range,
Specifically, vibration in the medium-frequency range, such as idling, typically has small amplitude of ±0.1-0.25 mm. As regards vibration in the low-frequency range, such as engine shake that is a problem during driving, there is now required effective vibration damping not only of large amplitude on the order of ±1.0 mm such as occurs when driving over speed bumps or the like, which has been considered a problem for some time, but also of small amplitude on the order of ±0.1 mm occurring during normal driving. Thus, as regards vibration of the low- to medium-frequency range having relatively small amplitude, if pressure fluctuations in the pressure receiving chamber are absorbed through small displacement of the movable plate, there is a risk that that fluid flow level through the first orifice passage or second orifice passage will be insufficient to afford adequate vibration damping action.